The invention relates to a method and arrangement for controlling a subscription in a mobile communication system. The invention finds particular utility in arranging prepaid calls and in monitoring a quantitative right of use in a digital mobile communication system.
The Subscriber Identity Module (SIM) card is an intelligent card used in digital mobile stations that stores, among other things, the subscriber""s identification information such as e.g. the mobile phone number. The system routes calls for the subscriber to that mobile station which has the subscriber""s SIM card installed in it. The subscriber may change mobile stations simply by removing the SIM card from one apparatus and inserting it in another one. In addition to said identification information the SIM card may store SMS (Short Message Service) messages, the subscriber""s private phone book as well as other information chosen by the subscriber. In addition, the GSM (Global System for Mobile Communications), for example, may store various control data in the SIM card. The SIM card and the architecture of the GSM are described in more detail e.g. in a book by Michel Mouly and Marie-Bernadette Pautet: The GSM System for Mobile Communications, ISBN 2-9507190-0-7, Palaiseau 1992. The SIM card is described in detail in the GSM Recommendation ETSI GSM 11.11 and in the standard ISO/IEC 7816.
When a person subscribes to a service provided by a mobile network operator, he or she is given a SIM card that identifies the subscription. Usually the operator charges the subscriber for the calls he or she has made using e.g. invoices sent to the subscriber regularly. Often, however, a person needs a mobile station for only a short period of time, e.g. in connection with a trip abroad, so it would be inconvenient to have a permanent subscription in that case. In many cases it would be too risky for the operator to allow credit to the subscriber, whereby payments in arrear cannot be allowed. For this reason there are mobile subscriptions in which a certain call charge is paid in advance, and if the prepaid sum is exhausted the mobile subscription will be closed. Such subscriptions are hereinafter called prepaid subscriptions.
In the current GSM system a prepaid subscription is realized such that information indicating the sum of prepaid call charges is stored in the SIM card. Use of a prepaid SIM card requires that messages be transferred between the SIM card and system so that the system can make sure the prepaid sum is not exceeded.
FIG. 1 shows signaling in relation to controlling the right of use of a subscription in a GSM system. A mobile station ME, as it becomes connected to a mobile network 10, is first authenticated. In the authentication, the network transmits to the mobile station 15 a random RAND number on the basis of which the SIM card 16 in the mobile station computes a so-called SRES number which is then transmitted back to the network. The SRES number is computed using the subscription identification information stored in the SIM card, so the network can verify the authenticity of the subscription by comparing the SRES number obtained from the mobile station with that computed in the network.
The SIM card 16 includes a so-called accumulated calling meter (ACM) register 161 such that the value in the register represents the cost accumulated from calls made on the mobile station. During a call the mobile station sends to the SIM card so-called INCREASE messages which, when received, increase the value in the ACM register stored in the SIM card. The value in the ACM register is increased according to the charge parameters transmitted by the network to the mobile station in the SETUP message in connection with call establishment. The charge parameters include e.g. the frequency at which the mobile station has to send INCREASE commands to the SIM card, and the amount with which the value in the ACM register has to be increased by each INCREASE command. Advice of Charge message is used to transmit the advance payment information of the call. In addition, a so-called ACMMAX value 162 is stored in the SIM card, which must not be exceeded by the value in the ACM register. As the value in the ACM register reaches the ACMMAX value, new calls cannot be made any more. When the user subsequently buys more call time for the SIM, the ACM register is reset and a possible new ACMMAX value is set.
The problem with the method described above is that the interface between the SIM card and mobile station is not protected, so the user may prevent the transmission of INCREASE commands to the SIM module. In that case the value in the ACM register will not be incremented and the user is able to place a limitless number of calls without paying for them.
A known solution to this problem is such that the ACM and ACMMAX register values are kept in the network instead of the SIM card. An advantage of this solution is its reliability and the fact that no registers are needed in the SIM card for the monitoring of the use of the mobile subscription. However, the problem with this solution is that during a call the network has to update the value in the ACM register continuously, which puts a considerable load on the network""s processing capacity.
In a second known solution the ACM registers are kept in the SIM card and the network transmits to the SIM card at predetermined moments of time an SMS (Short Message Service) message which contains an instruction to the SIM card to transmit the ACM register to the network. The SIM card interprets the instruction, reads the value in the ACM register, and sends to the network a response message containing the value of the ACM register. The network then compares the received ACM value with the previous received ACM value. If the ACM value has not changed even though calls have been placed from a mobile station using the subscriber""s SIM card, it is evident that the user has manipulated the interface between the SIM card and mobile station. In that case the network may terminate the subscription. An advantage of this solution is that the network need not update the ACM register during calls. A disadvantage of the solution is, however, that additional SMS signaling is needed for checking the ACM register.
The object of the invention is to provide a solution with which the use of a prepaid subscription can be controlled reliably without causing considerable need for signaling between the mobile station and network.
An idea of the invention is to include the information about the value of the parameter representing the amount of right of use in a response message sent to the network in connection with the authentication of the mobile station. Inclusion in the message can be realized using a predetermined algorithm. This way the network is able to check, using the corresponding algorithm, that the charge information stored in the SIM module is correct. Moreover, the algorithm stored in the SIM module can be altered by the operator e.g. by using the SMS message system associated with SIM modules.
The present invention is based on the use of an instruction, such as e.g. RUN GSM ALGORITHM, applied advantageously in the authentication of the subscription. The RUN GSM ALGORITHM instruction is sent to the SIM module in order to enable the use of the network. Furthermore, the response message SRES+Kc to the instruction has to be correct. If the user prevents the instruction from being transmitted to the mobile station the network will not receive a response from the mobile station in a predetermined time, in which case the use of the mobile station can be prevented.
An additional advantage of the invention is that no real-time mobile subscription monitoring is needed in the system""s switching equipment, so signaling and processing capacity in the system are saved for other needs.
A method according to the invention for controlling the right of use of a mobile subscription in a mobile communication system, in which the mobile subscription is authenticated by means of authentication signaling between a mobile station and the mobile communication system, is characterized in that information representing the spent and/or remaining amount of right of use is transmitted to the mobile communications system in connection with said authentication signaling.
A mobile communication system according to the invention, which comprises means for controlling the quantitative right of use of a mobile subscription and means for authenticating a mobile subscription by means of authentication signaling between a mobile station and the mobile communication system, is characterized in that the system also comprises means for transmitting information representing the spent and/or remaining amount of the right of use to the mobile communication system in connection with said authentication signaling.
A SIM card according to the invention, intended to be connected to a mobile station, comprising means for processing mobile subscription authentication signals and for generating an authentication message to be transferred to the mobile station and mobile communication system, is characterized in that it comprises means for including information representing the spent and/or remaining amount of the right of use of a mobile subscription in said authentication message.
A mobile station according to the invention is characterized in that it comprises a SIM card according to the invention.
The invention also pertains to a mobile station, which comprises means for connecting a SIM card to the mobile station and means for becoming connected to a mobile communication system, and means for exchanging mobile subscription authentication signals between the SIM card and mobile communication system, characterized in that the mobile station comprises means for transferring information representing the spent and/or remaining amount of a right of use from the SIM card to the mobile communication system in connection with said authentication signaling.
Preferred embodiments of the invention are presented in the dependent claims.
It should be noted that in this application the monitored amount relating to the charge or use of the mobile subscription can mean either the accumulated charge/use or the remaining charge/use. In this application, a SIM card refers generally to intelligent modules to identify a telephone subscription, thereby not being limited to any mechanical design used for the packing of the chips performing the functions, for example, nor limiting how the connections with the mobile station are realized.